THE "TABULATA." 147 



In the Aporosa the theca and septa are almost invariably 

 imperforate ; but, in the Perforata, they present apertures, 

 and, in some Madrepores, the whole skeleton is reduced to 

 a mere network of dense calcareous substance. When the 

 Hexacoralla multiply by gemmation or fission, and thus give 

 rise to compound massive or arborescent aggregations, each 

 newly-formed coral polyp develops a skeleton of its own, which 

 is either confluent with that of the others, or is united with 

 them by calcification of the connecting substance of the com- 

 mon body. This intermediate skeletal layer is then termed 

 coenenchyma. 



The septa in the adult Hexacoralla are often very numer- 

 ous and of different lengths, some approaching the centre 

 more closely than others do. Those of the same lengths are 

 members of one " cycle ; " and the cycles are numbered ac- 

 cording to the lengths of the septa, the longest being counted 

 as the first. In the young, six equal septa constitute the first 

 cycle. As the coral grows, another cycle of six septa arises 

 by the development of a new septum between each pair of 

 the first cycle ; and then a third cycle of twelve septa di- 

 vides the previously existing twelve interseptal chambers into 

 twenty-four. If we mark the septa of the first cycle A, those 

 of the second B, and those of the third C, then the space be- 

 tween any two septa (A A) of the first cycle will be thus rep- 

 resented when the third cycle is formed — A C B C A. 



When additional septa are developed, the fourth and fol- 

 lowing cycles do not consist of more than twelve septa each ; 

 hence the septa of each new cycle appear in twelve of the 

 previously existing interseptal spices, and not in all of them; 

 and the order of their appearance follows a definite law, which 

 has been worked out by Milne-Edwards and Haime. Thus, 

 the septa of the fourth cycle of twelve (d) bisect the inter- 

 septal space A C ; and those of the fifth cycle (e) the inter- 

 septal space B C ; the septa of the sixth cycle (f ), Ad and 

 d A ; those of thes eventh cycle (g), e B and B e ; those of the 

 eighth cycle (h), d C and C d; and those of the ninth cycle 

 (i), C e and e C. 



Hence, after the formation of nine cycles, the septa added 

 between everv pair of primary septa (A, A) will be thus ar- 

 ranged—A f d h C i e g B g e i h d f A. 1 



The stone-corals ordinarily known as Millepores are char- 



1 That the order of occurrence of the septa of various lengths, at the differ- 

 ent stages of growth of a corallite, is that indicated, seems to be clear, whatever 

 may be" the exact mode of development of the septa in each cycle. 



